Not applicable.
1. Field of the Invention
This invention relates to powder metal component parts, and in particular to a powder metal main bearing cap that reduces the total weight of an internal combustion engine or similar device.
2. Discussion of the Prior Art
There is a continual pressure to reduce vehicular weight in order to improve fuel efficiency and to conserve the world""s finite oil resources. Major advances have been made in this direction, including changing the engine cylinder block from cast iron to aluminum. Attention has turned to all the smaller incremental weight savings that are possible on every single component used in the engine.
It is necessary to retain an iron based (steel or cast iron) material for the main bearing cap that retains the crankshaft in its position at the lower end of the cylinder block. The iron base is needed since it has a matching thermal expansion rate with the crankshaft material, which is commonly either ductile cast iron or forged steel. In this way, as the engine temperature rises in service, the two components (bearing cap and crankshaft journal diameter) expand at the same rate, thereby maintaining constant running clearances at the bearing surfaces. However, since iron or steel is much more dense (about 300%) than aluminum, weight reduction of the iron based bearing cap is a prime target for weight reduction, especially since there are commonly 4 or 5 caps per engine.
It is known to make main bearing caps by the manufacturing process known as powder metallurgy. See, for example, International Patent Publication No. WO 97/42424. This process involves compaction of fine metallic and non-metallic powder blends in precision tooling that generate a precise powder preform or compact. This compact has low strength at this stage, only sufficient for handling without breaking as it is loaded onto a carrier plate that transports it through a thermal treatment furnace. This thermal treatment is known as xe2x80x9csinteringxe2x80x9d, and causes the powder particles to inter-diffuse and bond together to form a strong xe2x80x9csinteredxe2x80x9d component that has many functional engineering uses. There are many publications that explain the powder metallurgy (P/M) process in depth and practitioners of the art are knowledgeable in this process.
Although main bearing caps come in many different shapes, sizes and geometries, a main bearing cap 2 is typified by the design shown in FIG. 1. This consists of an essentially rectangular slab standing upon its long surface, with a half-circular section cut out from the lower side to create semi-circular surface 4 and legs 14 on each side of the surface 4. This form then has the general appearance of a xe2x80x9cbridgexe2x80x9d. The underbridge semi-circular surface 4 forms the lower half of a round bore when it is inverted and bolted to a mating semi-circular cut-out in the cylinder block. When the bearing caps are bolted to the cylinder block, they form the round-bore that holds the bearing shells that in turn hold the crankshaft in place. During the power stroke of the engine, a great load is placed upon the bearing caps that tries to separate the bearing caps from the cylinder block. The resistance to separation is provided by the main bearing bolts that clamp the cap to the cylinder block. As engine power has increased and cylinder block stiffness has decreased (aluminum is less stiff than cast iron), it has become the norm to use four main bolts, two through holes 6, 8 on each side of the bore which extend all of the way through the legs 14. Two more bolts are commonly provided at the outside ends of the cap, into blind tapped holes 10 formed in wings 15 (hole 10 in right wing 15 is present but not shown), to further stiffen the assembly (see FIG. 1).
The invention provides a design feature in a sintered powder metal main bearing cap, such as those used in combustion engines or similar reciprocating mechanical devices, which features a weight saving specially shaped slot between adjacent bolt holes on each side of the bearing arch. The connecting slot may extend the full length of the bolt holes, or for only the lower portion of the height, leaving a partial bridge to optimize under-bolt head clamping pressure. In addition, the outer profile of the bearing cap that surrounds the connected holes mimics the xe2x80x9chourglassxe2x80x9d shape so as to keep a uniform wall thickness around the slot region. The precise shape and size of the slot is calculated to ensure that the two bolts are guided into their respective tapped holes by the narrowing of the slot in an xe2x80x9chourglassxe2x80x9d shape that also maximizes the weight reduction from the metal removed by the slot form. The design and use of an hourglass shape section core-rod to form the connected holes is also an advantage during powder compaction.